A lithographic apparatus, such as a photolithographic apparatus, is a machine that applies a pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a device pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., a field including part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a set of adjacent target portions that are successively patterned.
In lithographic processes, it is desirable frequently to measure the structures created, e.g., for process control and monitoring. Various tools may make such measurements, including optical tools, like various forms of scatterometers. These devices typically direct a beam of radiation onto a target (e.g., a test structure on a patterned substrate) and measure properties of the scattered radiation. Examples of such properties include intensity of scattered radiation at a single angle of reflection as a function of wavelength; intensity at one or more wavelengths as a function of reflected angle; or polarization as a function of reflected angle. Measured properties often characterize a diffraction “spectrum” from which a property of interest of the target can be determined.